Field of the Invention
The present invention relates to photoacoustic apparatuses that utilizes photoacoustic effects.
Description of the Related Art
Photoacoustic imaging is one of the imaging techniques that utilize light. In photoacoustic imaging, a subject is irradiated with pulsed light generated by a light source. The irradiation light propagates and is scattered inside the subject and is absorbed at a plurality of sites inside the subject, and a photoacoustic wave is generated in turn. The photoacoustic wave is converted to an electric signal by a transducer, and the electric signal is analyzed and processed by a processing device. Thus, information pertaining to values indicative of optical characteristics inside the subject is acquired.
A generated sound pressure (hereinafter, also referred to as an initial sound pressure) P0 of a photoacoustic wave generated from a light absorber inside the subject can be expressed through the following expression.P0=Γ·μa·Φ  (1)
Herein, Γ represents the Gruneisen coefficient and is obtained by dividing the product of the coefficient of cubical expansion β and the square of the speed of sound c by the specific heat under constant pressure Cp. The symbol Φ represents the quantity of light at a given location (local region) (i.e., the quantity of light that has reached the absorber, which is also referred to as optical fluence).
The initial sound pressure P0 can be obtained by using a reception signal (PA signal) output from a probe that has received a photoacoustic wave.
It is known that the Gruneisen coefficient takes a substantially constant value for each given tissue, and thus it is possible to obtain the product of the optical absorption coefficient μa and the quantity of light Φ, or in other words, the optical energy absorption density by measuring and analyzing the change over time of the PA signals at a plurality of sites.
Japanese Patent Laid-Open No. 2013-248077 discloses a photoacoustic image generation apparatus that generates a photoacoustic image of a blood vessel on the basis of a photoacoustic wave generated from light.
In a case in which hemoglobin serves as a light absorber that is to be measured, the amount of hemoglobin is small in a region in which the blood volume within the blood vessel is small, and thus the optical absorption coefficient in that region is relatively low. Therefore, the sound pressure of the photoacoustic wave that is generated in accordance with the expression (1) tends to be low. In other words, the S/N ratio of a reception signal of a photoacoustic wave generated in a region in which the blood volume is small tends to be relatively low. Accordingly, when subject information of a target region is to be obtained by a photoacoustic apparatus, the accuracy of the acquired subject information may deteriorate in a region in which the blood volume is small.